1. Field of the Invention
The present invention is directed to an apparatus and method for exciting a turbine blade made of a material that does not respond to a magnetic field and, more particularly, to a system that attaches a small amount of material that is responsive to a magnetic field to the turbine blade.
2. Description of the Related Art
Free standing turbine blades are replacing the tenoned type blades used in steam generator turbines. Tenoned blades have tips riveted into segments of a ring and several segments are welded together to form a continuous ring. A free standing blade eliminates the need for riveting and welding and makes each blade simpler and less expensive to install. However, a free standing blade behaves like a complex tuning fork and has several modes of resonant vibrations. As a turbine is coming up to speed, it will pass through frequencies that will excite each blade at its resonant frequencies for brief periods. If turbine speed is maintained at one of the excitation frequencies or if a blade has a resonant frequency at or near the normal operating speed, the blade will likely fatigue and fail causing extensive damage to the turbine. As a result, it becomes essential that individual blades be tested to ensure that their resonant frequencies fall within allowable design limits. The measurement of stationary resonant frequency has become a very important measurement. Stationary resonant frequency has been conventionally measured using an accelerometer and mechanical means for exciting the blade. The mechanical exciters were difficult to use and did not produce consistently reproducible results because of adjustment problems.
Recently, a computerized test unit has been built to obtain more accurate stationary frequency test results and to improve the efficiency of the testing process. The computerized frequency test equipment incorporates the use of a non-contact electromagnetic means of exciting the blade and a non-contacting vibration pickup. This non-contact excitation and pickup system improves test repeatability and test efficiency.
This computerized prior art system works well with turbine blades that are made of a material that will respond to a magnetic field but will not work with blades made of a material such as titanium that will not respond to a magnetic field. Since free standing turbine blades are increasingly being made of magnetically unresponsive materials, the need has arisen for a system that will test resonant frequencies of turbine blades made from materials that will not respond to a magnetic field without using a mechanical exciter.